Monitoring system

ABSTRACT

Apparatus for performing impedance measurements on a subject. The apparatus includes a first processing system for determining an impedance measurement procedure and determining instructions corresponding to the measurement procedure. A second processing system is provided for receiving the instructions, using the instructions to generate control signals, with the control signals being used to apply one or more signals to the subject. The second processing system then receives first data indicative of the one or more signals applied to the subject, second data indicative of one or more signals measured across the subject and performs at least preliminary processing of the first and second data to thereby allow impedance values to be determined.

RELATED APPLICATIONS

This application is a U.S. National Phase under 35 U.S.C. 371 of theInternational Patent Application No. PCT/AU06/000922, filed Jun. 30,2006, and published in English on Jan. 11, 2007 as WO 2007/002991, whichclaims the benefit of U.S. Provisional Application No. 60/697,100, filedJul. 7, 2005, and Australian Application No. 2005903510, filed Jul. 1,2005.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for monitoringbiological parameters, and in particular to apparatus for makingimpedance measurements.

DESCRIPTION OF THE PRIOR ART

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that the prior publication (or information derived from it)or known matter forms part of the common general knowledge in the fieldof endeavour to which this specification relates.

One existing technique for determining biological parameters relating toa subject, such as cardiac function, involves the use of bioelectricalimpedance. This involves measuring the electrical impedance of asubject's body using a series of electrodes placed on the skin surface.Changes in electrical impedance at the body's surface are used todetermine parameters, such as changes in fluid levels, associated withthe cardiac cycle or oedema.

Accordingly, complex signal processing is required to ensuremeasurements can be interpreted.

Typically devices for achieving this utilise custom hardwareconfigurations that are application specific. As a result, the devicescan typically only be used in a limited range of circumstances.

SUMMARY OF THE PRESENT INVENTION

In a first broad form the present invention provides apparatus forperforming impedance measurements on a subject, the apparatus including:

-   -   a) a first processing system for:        -   i) determining an impedance measurement procedure; and,        -   ii) selecting instructions corresponding to the measurement            procedure; and,    -   b) a second processing system for:        -   i) generating, using the instructions, control signals, the            control signals being used to apply one or more signals to            the subject;        -   ii) receiving an indication of the one or more signals            applied to the subject;        -   iii) receiving an indication of one or more signals measured            across the subject;        -   iv) performing, using the instructions, at least preliminary            processing of the indications to thereby allow impedance            values to be determined.

Typically the method includes, transferring the instructions from thefirst processing system to the second processing system.

Typically the method includes, selecting the instructions usingconfiguration data.

Typically the method includes, receiving the configuration data from aremote processing system.

Typically the instructions are in the form of at least one of:

-   -   a) firmware; and,    -   b) embedded systems.

Typically the second processing system is an FPGA.

Typically the apparatus includes an input device, and wherein the firstprocessing system is coupled to the input device to thereby determinethe impedance measurement procedure in accordance with input commandsfrom an operator.

Typically the first processing system includes a store for storing atleast one profile, the at least one profile representing a predeterminedimpedance measurement procedure.

Typically the control signals represent a sequence of predeterminedelectrical signals, the sequence being dependent on the selectedimpedance measurement type.

Typically the apparatus includes:

-   -   a) a current ADC for:        -   i) receiving signals from a current circuit; and,        -   ii) providing the indication of the one or more signals            applied to the subject to the second processing system; and,    -   b) a voltage ADC for:        -   i) receiving signals from a voltage circuit; and,        -   ii) providing the indication of the one or more signals            measured from the subject to the second processing system.

Typically the apparatus includes at least one buffer circuit for:

-   -   a) receiving voltage signals from a voltage electrode;    -   b) filtering and amplifying the voltage signals; and,    -   c) transferring the filtered and amplified voltage signals to        the voltage ADC via a differential amplifier.

Typically the apparatus includes a current source circuit for:

-   -   a) receiving one or more control signals;    -   b) filtering and amplifying the control signals to thereby        generate one or more current signals;    -   c) applying the current signals to a current electrode; and,    -   d) transferring an indication of the applied signals to the        current ADC.

Typically the apparatus includes a control signal DAC for:

-   -   a) receiving the control signals from the second processing        system; and,    -   b) providing analogue control signals to a current circuit to        thereby cause one or more current signals to be applied to the        subject in accordance with the control signals.

Typically the second processing system is formed from first and secondprocessing system portions, the first and second processing systemportions being electrically isolated to thereby electrically isolate thesubject from the first processing system.

Typically the apparatus includes:

-   -   a) a measuring device including at least the first processing        system; and,    -   b) one or more subject units, each subject unit including at        least part of the second processing system.

Typically the apparatus includes at least two current electrodes forapplying current signals to the subject, and a switch connected to thecurrent electrodes for discharging the subject prior to measuring theinduced voltage.

Typically the apparatus includes a housing having:

-   -   a) a display;    -   b) a first circuit board for mounting at least one of the        processing systems;    -   c) a second circuit board for mounting at least one of an ADC        and a DAC; and,    -   d) a third circuit board for mounting a power supply.

Typically the housing is formed from at least one of a mu-metal andaluminium with added magnesium, to thereby provide electrical/magneticshielding.

Typically the apparatus includes multiple channels, each channel beingfor performing impedance measurements using a respective set ofelectrodes.

Typically the apparatus is for:

-   -   a) determining an electrode identifier associated with at least        one electrode provided on the subject;    -   b) determining, using the electrode identifier, an electrode        position indicative of the position of the at least one        electrode on the subject; and,    -   c) performing at least one impedance measurement using the        electrode position.

Typically the apparatus is for:

-   -   a) determining a parameter associated with at least one        electrode lead; and,    -   b) causing at least one impedance measurement to be performed        using the determined parameter.

Typically the apparatus is for:

-   -   a) receiving configuration data, the configuration data being        indicative of at least one feature;    -   b) determining, using the configuration data, instructions        representing the at least one feature; and,    -   c) causing, using the instructions, at least one of        -   i) at least one impedance measurement to be performed; and,        -   ii) at least one impedance measurement to be analysed.

Typically the apparatus is for:

-   -   a) causing a first signal to be applied to the subject;    -   b) determining at least one parameter relating to at least one        second signal measured across the subject;    -   c) comparing the at least one parameter to at least one        threshold; and,    -   d) depending on the results of the comparison, selectively        repeating steps (a) to (d) using a first signal having an        increased magnitude.

In a second broad form the present invention provides a method ofperforming impedance measurements on a subject, the method including:

-   -   a) using a first processing system for:        -   i) determining an impedance measurement procedure; and,        -   ii) selecting instructions corresponding to the measurement            procedure; and,    -   b) using a second processing system for:        -   i) generating, using the instructions, control signals, the            control signals being used to apply one or more signals to            the subject;        -   ii) receiving an indication of the one or more signals            applied to the subject;        -   iii) receiving an indication of one or more signals measured            across the subject;        -   iv) performing, using the instructions, at least preliminary            processing of the first and second data to thereby allow            impedance values to be determined.

In a third broad form the present invention provides a method ofdiagnosing conditions in a subject, the method including, in aprocessing system:

-   -   a) using a first processing system for:        -   i) determining an impedance measurement procedure; and,        -   ii) selecting instructions corresponding to the measurement            procedure; and,    -   b) using a second processing system for:        -   i) generating, using the instructions, control signals, the            control signals being used to apply one or more signals to            the subject;        -   ii) receiving an indication of the one or more signals            applied to the subject;        -   iii) receiving an indication of one or more signals measured            across the subject;        -   iv) performing, using the instructions, at least preliminary            processing of the first and second data to thereby allow            impedance values to be determined.

In a fourth broad form the present invention provides apparatus forconnecting measurement apparatus to an electrode, the apparatusincluding:

-   -   a) a housing having a connector for coupling the housing to an        electrode; and,    -   b) a circuit mounted in the housing, the circuit being        electrically coupled to the electrode using the connector, and        being coupled to a lead, the circuit being for at least one of        -   i) generating predetermined electrical signals in accordance            with control signals received from the measurement            apparatus;        -   ii) providing an indication of electrical signals applied to            the electrode; and,        -   iii) providing an indication of electrical signals measured            at the electrode.

Typically the circuit is provided on a circuit board having anelectrical contact, and wherein in use the connector urges at least partof the electrode into abutment with the electrical contact.

Typically the connector includes a biased arm.

Typically the circuit includes a buffer circuit for:

-   -   a) sensing voltage signals at the electrode;    -   b) filtering and amplifying the voltage signals; and,    -   c) transferring the filtered and amplified voltage signals to        the measurement apparatus.

Typically the circuit includes a current source circuit for:

-   -   a) receiving one or more control signals;    -   b) filtering and amplifying the control signals to thereby        generate one or more current signals;    -   c) applying the current signals to the electrode pad; and,    -   d) transferring an indication of the applied signals to the        measurement apparatus.

Typically the apparatus further comprises an electrode, the electrodeincluding:

-   -   a) an electrode substrate; and,    -   b) a conductive material for electrically coupling the electrode        to the subject.

Typically the electrode substrate is electrically conductive, andwherein in use the connector couples the circuit to the electrodesubstrate.

Typically the housing includes curved edges.

Typically the housing is formed from a material that, at least one of

-   -   a) has a low coefficient of friction; and,    -   b) is resilient.

In a fifth broad form the present invention provides a method ofperforming impedance measurements on a subject, the method including, ina processing system:

-   -   a) determining an encoded value associated with at least one        electrode lead; and,    -   b) causing at least one impedance measurement to be performed        using the encoded value.

Typically the encoded value is used for calibration.

Typically the encoded value is determined from a resistance value.

Typically the encoded value is indicative of an identity of the lead.

Typically the method includes, in the processing system, controlling thecurrent applied to the subject using the determined resistance.

Typically the encoded value is a lead identifier, and wherein the methodincludes, in the processing system:

-   -   a) determining, using the lead identifier, an impedance        measurement procedure; and,    -   b) causing the determined impedance measurement procedure to be        performed.

Typically the method includes, in the processing system:

-   -   a) comparing the determined identity to one or more        predetermined identities; and,    -   b) determining the impedance of the subject in response to a        successful comparison.

Typically the method includes, in the processing system:

-   -   a) determining the lead identifier associated with the at least        one electrode lead;    -   b) determining, using the lead identifier, a lead usage;    -   c) comparing the lead usage to a threshold; and,    -   d) in accordance with the results of the comparison, at least        one of:        -   i) generating an alert;        -   ii) terminating an impedance measurement procedure; and,        -   iii) performing an impedance measurement procedure.

Typically the method includes, in the processing system, at least oneof:

-   -   a) processing electrical signals measured from the subject to        thereby determine one or more impedance values; and,    -   b) processing determined impedance values.

Typically the encoded value is stored in a store.

In a sixth broad form the present invention provides apparatus forperforming impedance measurements on a subject, the apparatus including:

-   -   a) at least one lead for connecting to electrodes coupled to the        subject, the at least one lead including an encoded value; and,    -   b) a processing system coupled to the at least one lead for:        -   i) determining the encoded value; and,    -   c) causing at least one impedance measurement to be performed        using the encoded value.

In a seventh broad form the present invention provides a method ofperforming impedance measurements on a subject, the method including, ina processing system:

-   -   a) determining an electrode identifier associated with at least        one electrode provided on the subject;    -   b) determining, using the electrode identifier, an electrode        position indicative of the position of the at least one        electrode on the subject; and,    -   c) causing at least one impedance measurement to be performed        using the electrode position.

Typically the impedance measurement is performed using at least fourelectrodes, each having a respective identifier, and wherein the methodincludes, in the processing system:

-   -   a) determining an electrode identifier for each electrode;    -   b) determining, using each electrode identifier, an electrode        position for each electrode; and,    -   c) performing at least one impedance measurement using the        electrode positions.

Typically the method includes, in the processing system:

-   -   a) causing signals to be applied to at least two of the        electrodes in accordance with the determined electrode        positions; and,    -   b) causing signals to be measured from at least two of the        electrodes in accordance with the determined electrode        positions.

Typically the method includes, in the processing system, determining theelectrode identifier for an electrode by selectively measuring theconductivity between one or more contacts provided on the electrode.

Typically the processing system is coupled to a signal generator and asensor, and wherein the method includes, in the processing system:

-   -   a) selectively interconnecting the signal generator and at least        two electrode leads, to thereby allow signals to be applied to        the subject; and,    -   b) selectively interconnecting the sensor at least two electrode        leads to thereby allow a signal to be measured from the subject.

Typically the method includes, in the processing system controlling amultiplexer to thereby selectively interconnect the leads and at leastone of the signal generator and the sensor.

Typically the at least one electrode includes visual indicia indicativeof the position of the at least one electrode on the subject.

In an eighth broad form the present invention provides apparatus forperforming impedance measurements on a subject, the apparatus includinga processing system for:

-   -   a) determining an electrode identifier associated with at least        one electrode provided on the subject;    -   b) determining, using the electrode identifier, an electrode        position indicative of the position of the at least one        electrode on the subject; and,    -   c) causing at least one impedance measurement to be performed        using the electrode position.

In a ninth broad form the present invention provides a method forconfiguring a measuring device for measuring the impedance of a subject,the method including, in a processing system:

-   -   a) receiving configuration data, the configuration data being        indicative of at least one feature;    -   b) determining, using the configuration data, instructions        representing the at least one feature; and,    -   c) causing, at least in part using the instructions, at least        one of:        -   i) impedance measurements to be performed; and,        -   ii) analysis of impedance measurements.

Typically the configuration data includes the instructions.

Typically the method includes, in the processing system:

-   -   a) determining an indication of the at least one feature using        the configuration data; and,    -   b) determining the instructions using the indication of the at        least one feature.

Typically the method includes, in the processing system, decrypting thereceived configuration data.

Typically the method includes, in the processing system:

-   -   a) determining a device identifier associated with the measuring        device;    -   b) determining, using the device identifier, a key; and,    -   c) decrypting the received configuration data using the key.

Typically the processing system includes first and second processingsystems, and wherein the method includes:

-   -   a) in the first processing system, selecting the instructions        using the configuration data; and,    -   b) in the second processing system, generating the control        signals using selected instructions.

Typically the method includes, in the processing first system, at leastone of:

-   -   a) transferring the instructions to the second processing        system; and,    -   b) causing the second processing system to access the        instructions from a store.

Typically the method includes, in the processing system, receiving theconfiguration data from at least one of a computer system and acommunications network.

Typically the method includes, in the processing system:

-   -   a) determining if a feature selected by a user is available;    -   b) if the feature is not available, determining if the user        wishes to enable the feature; and,    -   c) if the user wishes to enable the feature, causing        configuration data to be received.

Typically the method includes, in the processing system:

-   -   a) causing the user to provide a payment to a device provider;        and,    -   b) receiving the configuration data in response to payment.

In a tenth broad form the present invention provides apparatus forconfiguring a measuring device for measuring the impedance of a subject,the apparatus including a processing system for:

-   -   a) receiving configuration data, the configuration data being        indicative of at least one feature;    -   b) determining, using the configuration data, instructions        representing the at least one feature; and,    -   c) causing, at least in part using the instructions, at least        one of:        -   i) impedance measurements to be performed; and,        -   ii) analysis of impedance measurements.

In an eleventh broad form the present invention provides a method forconfiguring a measuring device for measuring the impedance of a subject,the method including, in a computer system:

-   -   a) determining configuration data required for a measuring        device, the configuration data being indicative of at least one        feature; and,    -   b) causing the configuration data to be received by a processing        system in the measuring device, the processing system being        responsive to the configuration data to configure the measuring        device to allow the at least one feature to be used.

Typically the method includes, in the computer system:

-   -   a) determining a device identifier, the device identifier being        associated with the measuring device to be configured; and,    -   b) using the device identifier to at least one of:        -   i) transfer the configuration data to the measuring device;            and,        -   ii) encrypt the configuration data.

Typically the method includes, in the computer system, determining theconfiguration data is required in response to at least one of

-   -   a) payment made by a user of the measuring device; and,    -   b) approval of the feature.

Typically the method includes, in the computer system:

-   -   a) determining regulatory approval of the at least one feature        in at least one region;    -   b) determining at least one measuring device in the at least one        region; and,    -   c) configuring the at least one measuring device.

In a twelfth broad form the present invention provides apparatus forconfiguring a measuring device for measuring the impedance of a subject,the method including, in a computer system:

-   -   a) determining configuration data required for a measuring        device, the configuration data being indicative of at least one        feature; and,    -   b) causing the configuration data to be received by a processing        system in the measuring device, the processing system being        responsive to the configuration data to configure the measuring        device to allow the at least one feature to be used.

In a thirteenth broad form the present invention provides a method ofperforming impedance measurements on a subject, wherein the methodincludes, in a processing system:

-   -   a) causing a first signal to be applied to the subject;    -   b) determining at least one parameter relating to at least one        second signal measured across the subject;    -   c) comparing the at least one parameter to at least one        threshold; and,    -   d) depending on the results of the comparison, selectively        repeating steps (a) to (d) using a first signal having an        increased magnitude.

Typically the method includes, in the processing system:

-   -   a) determining an animal type of the subject; and,    -   b) selecting the threshold in accordance with the animal type.

Typically the threshold is indicative of at least one of:

-   -   a) a minimum second signal magnitude; and,    -   b) a minimum signal to noise ratio for the second signal.

Typically the method includes, in the processing system:

-   -   a) determining at least one parameter relating to the at least        one first signal;    -   b) comparing the at least one parameter to at least one        threshold; and,    -   c) selectively terminating impedance measurements depending on        the results of the comparison.

Typically the threshold is indicative of a maximum first signalmagnitude.

In a fourteenth broad form the present invention provides apparatus forperforming impedance measurements on a subject, wherein the apparatusincludes a processing system for:

-   -   a) causing a first signal to be applied to the subject;    -   b) determining at least one parameter relating to at least one        second signal measured across the subject;    -   c) comparing the at least one parameter to at least one        threshold; and,    -   d) depending on the results of the comparison, selectively        repeating steps (a) to (d) using a first signal having an        increased magnitude.

Typically the apparatus further includes a variable magnitude currentsupply.

In another broad form the present invention provides a method ofproviding an electrode for use in impedance measurement procedures, themethod including:

-   -   a) providing on a substrate:        -   i) a number of electrically conductive contact pads; and,        -   ii) a corresponding number of electrically conductive            tracks, each track extending from an edge of the substrate            to a respective contact pad;    -   b) applying an insulating layer to the substrate, the insulating        layer including a number of apertures, and being positioned to        thereby overlay the tracks with at least a portion of each pad        contact aligned with a respective aperture; and,    -   c) providing an electrically conductive medium in the apertures.

Typically the electrically conductive medium is formed from a conductivegel.

Typically the conductive gel is silver/silver chloride gel.

Typically the method includes, providing a covering layer on theinsulating layer to thereby cover the electrically conductive medium.

Typically the insulating layer has an adhesive surface that releasablyengages the covering layer.

Typically the substrate is an elongate substrate, and wherein the methodincludes aligning the pad contacts along the length of the substrate.

Typically the method includes providing the tracks and contact padsusing at least one of

-   -   a) screen printing;    -   b) inkjet printing; and,    -   c) vapour deposition.

Typically the tracks and contact pads are formed from silver.

Typically the method includes forming the substrate by:

-   -   a) overlaying a plastic polymer with a shielding material; and,    -   b) covering the shielding material with an insulating material.

In a fifteenth broad form the present invention provides an electrodefor use in impedance measurement procedures, the electrode including:

-   -   a) a substrate having provided thereon:        -   i) a number of electrically conductive contact pads; and,        -   ii) a corresponding number of electrically conductive            tracks, each track extending from an edge of the substrate            to a respective contact pad;    -   b) an insulating layer provided on the substrate, the insulating        layer including a number of apertures, and being positioned to        thereby overlay the tracks with at least a portion of each pad        contact aligned with a respective aperture; and,    -   c) an electrically conductive medium provided in the apertures.

In a sixteenth broad form the present invention provides a method foruse in diagnosing conditions in a subject, the method including, in aprocessing system:

-   -   a) determining an encoded value associated with at least one        electrode lead; and,    -   b) causing at least one impedance measurement to be performed        using the encoded value.

In a seventeenth broad form the present invention provides a method foruse in diagnosing conditions in a subject, the method including, in aprocessing system:

-   -   a) determining an electrode identifier associated with at least        one electrode provided on the subject;    -   b) determining, using the electrode identifier, an electrode        position indicative of the position of the at least one        electrode on the subject; and,    -   c) causing at least one impedance measurement to be performed        using the electrode position.

In an eighteenth broad form the present invention provides a method foruse in diagnosing conditions in a subject, the method including, in aprocessing system:

-   -   a) receiving configuration data, the configuration data being        indicative of at least one feature;    -   b) determining, using the configuration data, instructions        representing the at least one feature; and,    -   c) causing the measuring device to perform, using the        instructions, at least one of:        -   i) impedance measurements; and,        -   ii) analysis of impedance measurements.

In a nineteenth broad form the present invention provides a method foruse in diagnosing conditions in a subject, the method including, in aprocessing system:

-   -   a) determining configuration data required for a measuring        device, the configuration data being indicative of at least one        feature; and,    -   b) causing the configuration data to be received by a processing        system in the measuring device, the processing system being        responsive to the configuration data to configure the measuring        device to allow the at least one feature to be used.

In a twentieth broad form the present invention provides a method foruse in diagnosing conditions in a subject, the method including, in aprocessing system:

-   -   a) causing a first signal to be applied to the subject;    -   b) determining at least one parameter relating to at least one        second signal measured across the subject;    -   c) comparing the at least one parameter to at least one        threshold; and,    -   d) depending on the results of the comparison, selectively        repeating steps (a) to (d) using a first signal having an        increased magnitude.

It will be appreciated that the broad forms of the invention may be usedindividual or in combination, and may be used for diagnosis of thepresence, absence or degree of a range of conditions and illnesses,including, but not limited to oedema, pulmonary oedema, lymphodema, bodycomposition, cardiac function, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the present invention will now be described with referenceto the accompanying drawings, in which:—

FIG. 1 is a schematic of an example of impedance determinationapparatus;

FIG. 2 is a flowchart of an example of a process for performingimpedance determination;

FIG. 3 is a schematic of a second example impedance determinationapparatus;

FIG. 4 is a schematic of an example of a current source circuit;

FIG. 5 is a schematic of an example of a buffer circuit for use involtage sensing;

FIGS. 6A and 6B is a flowchart of a second example of a process forperforming impedance determination;

FIGS. 7A and 7B are schematics of an example of an electrode connection;

FIG. 8 is a schematic of a third example of impedance determinationapparatus;

FIG. 9 is a schematic of a fourth example of impedance determinationapparatus; and,

FIG. 10 is a schematic of a fifth example of impedance determinationapparatus;

FIGS. 11A and 11B are schematic diagrams of a second example of anelectrode connection;

FIGS. 11C to 11G are schematic diagrams of a third example of anelectrode connection;

FIGS. 12A to 12F are schematic diagrams of an example of theconstruction of a band electrode;

FIGS. 12G and 12H are schematic diagrams of an example of a connectorarrangement for the band electrode;

FIG. 12I is a schematic diagram of the use of a band electrode;

FIG. 13 is a schematic of a second example of a current source circuit;

FIG. 14 is a flow chart of an example of using the current sourcecircuit of FIG. 13;

FIG. 15 is a flow chart of an overview of an example of the process ofupdating a measuring device;

FIG. 16 is a schematic diagram of an example of a system architecturefor updating a measuring device;

FIG. 17 is a flow chart of a first example of the process of updating ameasuring device;

FIG. 18 is a flow chart of a second example of the process of updating ameasuring device; and,

FIG. 19 is a schematic of an example of a housing configuration forimpedance determination apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of apparatus suitable for performing an analysis of asubject's bioelectric impedance will now be described with reference toFIG. 1.

As shown the apparatus includes a measuring device 1 including aprocessing system 2 coupled to a signal generator 11 and a sensor 12. Inuse the signal generator 11 and the sensor 12 are coupled to respectiveelectrodes 13, 14, 15, 16, provided on a subject S, via leads L, asshown. An optional external interface 23 can be used to couple themeasuring device 1 to one or more peripheral devices 4, such as anexternal database or computer system, barcode scanner, or the like.

In use, the processing system 2 is adapted to generate control signals,which causes the signal generator 11 to generate one or more alternatingsignals, such as voltage or current signals, which can be applied to asubject S, via the electrodes 13, 14. The sensor 12 then determines thevoltage across or current through the subject S, using the electrodes15, 16 and transfers appropriate signals to the processing system 2.

Accordingly, it will be appreciated that the processing system 2 may beany form of processing system which is suitable for generatingappropriate control signals and interpreting an indication of themeasured signals to thereby determine the subject's bioelectricalimpedance, and optionally determine other information such as thecardiac parameters, presence absence or degree of oedema, or the like.

The processing system 2 may therefore be a suitably programmed computersystem, such as a laptop, desktop, PDA, smart phone or the like.Alternatively the processing system 2 may be formed from specialisedhardware. Similarly, the I/O device may be of any suitable form such asa touch screen, a keypad and display, or the like.

It will be appreciated that the processing system 2, the signalgenerator 11 and the sensor 12 may be integrated into a common housingand therefore form an integrated device. Alternatively, the processingsystem 2 may be connected to the signal generator 11 and the sensor 12via wired or wireless connections. This allows the processing system 2to be provided remotely to the signal generator 11 and the sensor 12.Thus, the signal generator 11 and the sensor 12 may be provided in aunit near, or worn by the subject S, whilst the processing system 2 issituated remotely to the subject S.

In one example, the outer pair of electrodes 13, 14 are placed on thethoracic and neck region of the subject S. However, this depends on thenature of the analysis being performed. Thus, for example, whilst thiselectrode arrangement is suitable for cardiac function analysis, inlymphoedema, the electrodes would typically be positioned on the limbs,as required.

Once the electrodes are positioned, an alternating signal is applied tothe subject S. This may be performed either by applying an alternatingsignal at a plurality of frequencies simultaneously, or by applying anumber of alternating signals at different frequencies sequentially. Thefrequency range of the applied signals may also depend on the analysisbeing performed.

In one example, the applied signal is a frequency rich current from acurrent source clamped, or otherwise limited, so it does not exceed themaximum allowable subject auxiliary current. However, alternatively,voltage signals may be applied, with a current induced in the subjectbeing measured. The signal can either be constant current, impulsefunction or a constant voltage signal where the current is measured soit does not exceed the maximum allowable subject auxiliary current.

A potential difference and/or current are measured between an inner pairof electrodes 15, 16. The acquired signal and the measured signal willbe a superposition of potentials generated by the human body, such asthe ECG, and potentials generated by the applied current.

Optionally the distance between the inner pair of electrodes may bemeasured and recorded. Similarly, other parameters relating to thesubject may be recorded, such as the height, weight, age, sex, healthstatus, any interventions and the date and time on which they occurred.Other information, such as current medication, may also be recorded.

To assist accurate measurement of the impedance, buffer circuits may beplaced in connectors that are used to connect the voltage sensingelectrodes 15, 16 to the leads L. This ensures accurate sensing of thevoltage response of the subject S, and in particular helps eliminatecontributions to the measured voltage due to the response of the leadsL, and reduce signal loss.

This in turn greatly reduces artefacts caused by movement of the leadsL, which is particularly important during dialysis as sessions usuallylast for several hours and the subject will move around and changepositions during this time.

A further option is for the voltage to be measured differentially,meaning that the sensor used to measure the potential at each electrode15, 16 only needs to measure half of the potential as compared to asingle ended system.

The current measurement system may also have buffers placed in theconnectors between the electrodes 13, 14 and the leads L. In oneexample, current can also be driven or sourced through the subject Ssymmetrically, which again greatly reduced the parasitic capacitances byhalving the common-mode current. Another particular advantage of using asymmetrical system is that the micro-electronics built into theconnectors for each electrode 13, 14 also removes parasitic capacitancesthat arise when the subject S, and hence the leads L move.

The acquired signal is demodulated to obtain the impedance of the systemat the applied frequencies. One suitable method for demodulation ofsuperposed frequencies is to use a Fast Fourier Transform (FFT)algorithm to transform the time domain data to the frequency domain.This is typically used when the applied current signal is asuperposition of applied frequencies. Another technique not requiringwindowing of the measured signal is a sliding window FFT.

In the event that the applied current signals are formed from a sweep ofdifferent frequencies, then it is more typical to use a processingtechnique such as multiplying the measured signal with a reference sinewave and cosine wave derived from the signal generator, or with measuredsine and cosine waves, and integrating over a whole number of cycles.This process rejects any harmonic responses and significantly reducesrandom noise.

Other suitable digital and analog demodulation techniques will be knownto persons skilled in the field.

Impedance or admittance measurements are determined from the signals ateach frequency by comparing the recorded voltage and current signal. Thedemodulation algorithm will produce an amplitude and phase signal ateach frequency.

An example of the operation of the apparatus for performing impedanceanalysis will now be described with reference to FIG. 2.

At step 100, the processing system 2 operates to generate controlsignals which are provided to the signal generator 11 at step 110,thereby causing the signal generator to apply an alternating currentsignal to the subject S, at step 120. Typically the signal is applied ateach of a number of frequencies f_(i) to allow multiple frequencyanalysis to be performed.

At step 130 the sensor 12 senses voltage signals across the subject S.At step 140 the measuring device, operates to digitise and sample thevoltage and current signals across the subject S, allowing these to beused to determine instantaneous impedance values for the subject S atstep 150.

A specific example of the apparatus will now be described in more detailwith respect to FIG. 3. In this example, the processing system 2includes a first processing system 10 having a processor 20, a memory21, an input/output (I/O) device 22, and an external interface 23,coupled together via a bus 24. The processing system 2 also includes asecond processing system 17, in the form of a processing module. Acontroller 19, such as a micrologic controller, may also be provided tocontrol activation of the first and second processing systems 10, 17.

In use, the first processing system 10 controls the operation of thesecond processing system 17 to allow different impedance measurementprocedures to be implemented, whilst the second processing system 17performs specific processing tasks, to thereby reduce processingrequirements on the first processing system 10.

Thus, the generation of the control signals, as well as the processingto determine instantaneous impedance values is performed by the secondprocessing system 17, which may therefore be formed from customhardware, or the like. In one particular example, the second processingsystem 17 is formed from a Field Programmable Gate Array (FPGA),although any suitable processing module, such as a magnetologic module,may be used.

The operation of the first and second processing systems 10, 17, and thecontroller 19 is typically controlled using one or more sets ofappropriate instructions. These could be in any suitable form, and maytherefore include, software, firmware, embedded systems, or the like.

The controller 19 typically operates to detect activation of themeasuring device through the use of an on/off switch (not shown). Oncethe controller detects device activation, the controller 19 executespredefined instructions, which in turn causes activation of the firstand second processing systems 10, 17, including controlling the supplyof power to the processing systems as required.

The first processing system 10 can then operate to control theinstructions, such as the firmware, implemented by the second processingsystem 17, which in turn alters the operation of the second processingsystem 17. Additionally, the first processing system 10 can operate toanalyse impedance determined by the second processing system 17, toallow biological parameters to be determined.

Accordingly, the first processing system 10 may be formed from customhardware or the like, executing appropriate applications software toallow the processes described in more detail below to be implemented.

It will be appreciated that this division of processing between thefirst processing system 10, and the second processing system 17, is notessential, but there are a number of benefits that will become apparentfrom the remaining description.

In this example, the second processing system 17 includes a PCI bridge31 coupled to programmable module 36 and a bus 35, as shown. The bus 35is in turn coupled to processing modules 32, 33, 34, which interfacewith ADCs (Analogue to Digital Converters) 37, 38, and a DAC (Digital toAnalogue Converter) 39, respectively.

The programmable module 36 is formed from programmable hardware, theoperation of which is controlled using the instructions, which aretypically downloaded from the first processing system 10. The firmwarethat specifies the configuration of hardware 36 may reside in flashmemory (not shown), in the memory 21, or may be downloaded from anexternal source via the external interface 23.

Alternatively, the instructions may be stored within inbuilt memory onthe second processing system 17. In this example, the first processingsystem 10 typically selects firmware for implementation, before causingthis to be implemented by the second processing system 17. This may beachieved to allow selective activation of functions encoded within thefirmware, and can be performed for example using configuration data,such as a configuration file, or instructions representing applicationssoftware or firmware, or the like, as will be described in more detailbelow.

In either case, this allows the first processing system 10 to be used tocontrol operation of the second processing system 17 to allowpredetermined current sequences to be applied to the subject S. Thus,for example, different firmware would be utilised if the current signalis to be used to analyse the impedance at a number of frequenciessimultaneously, for example, by using a current signal formed from anumber of superposed frequencies, as compared to the use of currentsignals applied at different frequencies sequentially.

An example of a specific form of signal generator 11 in the form of acurrent source circuit, is shown in FIG. 4.

As shown the current source includes three fixed or variable gaindifferential amplifiers A₁, A₂, A₃ and three op-amps A₄, A₅, A₆, anumber of resistors R₁, . . . R₁₇ and capacitors C₁, . . . C₄,interconnected as shown. The current source also includes leads 41, 42(corresponding to the leads L in FIG. 1) which connect the currentsource to the electrodes 13, 14 and a switch SW for shorting the leads41, 42 as will be described in more detail below.

Connections 45, 46 can also be provided for allowing the current appliedto the subject S to be determined. Typically this is achieved using theconnection 46. However, the connection 45 may also be used as shown indotted lines to allow signal losses within the leads and other circuitryto be taken into account.

In general the leads used are co-axial cables with a non-braided shieldand a multi strand core with a polystyrene dielectric. This providesgood conductive and noise properties as well as being sufficientlyflexible to avoid issues with connections from the measuring device 1 tothe subject S. In this instance, resistors R₁₂, R₁₃ decouple the outputsof the amplifiers A₅, A₆ from the capacitances associated with cable.

In use, the current source circuit receives current control signals I⁺,I⁻ from the DAC 39, with these signals being filtered and amplified, tothereby form current signals that can be applied to the subject S viathe electrodes 13, 14.

In use, when the amplifiers A₁, . . . A₆ are initially activated, thiscan lead to a minor, and within safety limits, transient current surge.As the current is applied to the subject, this can result in thegeneration of a residual field across the subject S. To avoid this fieldeffecting the readings, the switch SW is generally activated prior tomeasurements being taken, to short the current circuit, and therebydischarge any residual field.

Once the measurement is commenced, an indication of the current appliedto the subject can be obtained via either one of the connections 45, 46,that are connected to the ADC 38, as shown by the dotted lines.

This allows the current supplied across the subject to be accuratelydetermined. In particular, by using the actual applied current, asopposed to estimating the current applied on the basis of the controlsignals I⁺, I⁻, this takes into account non-ideal behaviour of thecomponents in the current source, and can also take into account theeffects of the leads 41, 42, on the applied current.

In one example, the amplifier A₃ and associated components may beprovided on a housing coupled to the electrodes 12, 13, allowing moreaccurate sensing of the current applied to the subject. In particular,this avoids measuring of cable effects, such as signal loss in the leadsL.

The above is an example of a non-symmetric current source and it will beappreciated that symmetric current sources may alternatively be used.

An example of the buffer used for the voltage electrodes is shown inFIG. 5. In this example, each electrode 15, 16, will be coupled to abuffer circuit 50A, 50B.

In this example, each buffer 50A, 50B includes amplifiers A₁₀, A₁₁, anda number of resistors R₂₁, . . . , R₂₆, interconnected as shown. In use,each buffer 50A, 50B, is connected a respective electrode 15, 16 viaconnections 51, 52. The buffers 50A, 50B are also connected via leads53, 54 to a differential amplifier 55, acting as the signal sensor 12,which is in turn coupled to the ADC 37. It will therefore be appreciatedthat a respective buffer circuit 50A, 50B is connected to each of theelectrodes 15, 16, and then to a differential amplifier, allowing thepotential difference across the subject to be determined.

In one example, the leads 53, 54 correspond to the leads L shown in FIG.1, allowing the buffer circuits 50A, 50B to be provided in connectorhousing coupled to the electrodes 15, 16, as will be described in moredetail below.

In use, the amplifier A₁₀ amplifies the detected signals and drives thecore of the cable 53, whilst the amplifier A₁₁ amplifies the detectedsignal and drives the shield of the cables 51, 53. Resistors R₂₆ and R₂₅decouple the amplifier outputs from the capacitances associated withcable, although the need for these depends on the amplifier selected.

Again, this allows multi-core shielded cables to be used to establishthe connections to the voltage electrodes 15, 16.

An example of operation of the apparatus will now be described withreference to FIGS. 6A to 6C.

At step 200 an operator selects an impedance measurement type using thefirst processing system 10. This may be achieved in a number of ways andwill typically involve having the first processing system 10 store anumber of different profiles, each of which corresponds to a respectiveimpedance measurement protocol.

Thus, for example, when performing cardiac function determination, itwill be typical to use a different applied current sequence and adifferent impedance analysis, as compared to performing lymphoedemameasurements, body composition, pulmonary oedema, or the like. Theprofile will typically be stored in the memory 21, or alternatively maybe downloaded from flash memory (not shown), or via the externalinterface 23.

Once an appropriate measurement type has been selected by the operator,this will cause the first processing system 10 to load desired codemodule firmware into the programmable module 36 of the second processingsystem 17 at step 210, or cause embedded firmware to be activated. Thetype of code module used will depend on the preferred implementation,and in one example this is formed from a wishbone code module, althoughthis is not essential.

At step 220, the second processing system 17 is used to generate asequence of digital control signals, which are transferred to the DAC 39at step 230. This is typically achieved using the processing module 34,by having the module generate a predetermined sequence of signals basedon the selected impedance measurement profile. This can therefore beachieved by having the second processing system 17 program theprocessing module 34 to cause the module to generate the requiredsignals.

The DAC 39 converts the digital control signals into analogue controlsignals I⁺, I⁻ which are then applied to the current source 11 at step240.

As described above, the current source circuit shown in FIG. 4 operatesto amplify and filter the electrical control signals I⁺, I⁻ at step 250,applying the resulting current signals to the electrodes 13, 14 at step260.

During this process, and as mentioned above, the current circuit throughthe subject can optionally be shorted at step 270, using the switch SW,to thereby discharge any residual field in the subject S, prior toreadings being made.

At step 280, the measurement procedure commences, with the voltageacross the subject being sensed from the electrodes 15, 16. In thisregard, the voltage across the electrodes is filtered and amplifiedusing the buffer circuit shown in FIG. 5 at step 290, with the resultantanalogue voltage signals V being supplied to the ADC 37 and digitised atstep 300. Simultaneously, at step 310 the current applied to the subjectS is detected via one of the connections 45, 46, with the analoguecurrent signals I being digitised using the ADC 38 at step 320.

The digitised voltage and current signals V, I are received by theprocessing modules 32, 33 at step 330, with these being used toperformed preliminary processing of the signals at step 340.

The processing performed will again depend on the impedance measurementprofile, and the consequent configuration of the processing modules 32,33. This can include for example, processing the voltage signals V toextract ECG signals. The signals will also typically be filtered toensure that only signals at the applied frequencies are used inimpedance determination. This helps reduce the effects of noise, as wellas reducing the amount of processing required.

At step 350 the second processing system 17 uses the processing signalsto determine voltage and current signals at each applied frequencyf_(i), with these being used at step 360 to determine instantaneousimpedance values at each applied frequency f_(i).

The ADCs 37, 38 and the processing modules 32, 33 are typically adaptedto perform sampling and processing of the voltage and current signals V,I in parallel so that the voltage induced at the corresponding appliedcurrent are analysed simultaneously. This reduces processingrequirements by avoiding the need to determine which voltage signalswere measured at which applied frequency.

This is achieved by having the processing modules 32, 33 sample thedigitised signals received from the ADCs 37, 38, using a common clocksignal generated by the processing module 36, which thereby ensuressynchronisation of the signal sampling.

Once the instantaneous impedance values have been derived, these canundergo further processing in either the first processing system 10, orthe second processing system 17, at step 370. The processing of theinstantaneous impedance signals will be performed in a number ofdifferent manners depending on the type of analysis to be used and thisin turn will depend on the selection made by the operator at step 200.

Accordingly, it will be appreciated by persons skilled in the art that arange of different current sequences can be applied to the subject bymaking an appropriate measurement type selection. Once this has beenperformed, the FPGA operates to generate a sequence of appropriatecontrol signals I⁺, I⁻, which are applied to the subject S using thecurrent supply circuit shown in FIG. 4. The voltage induced across thesubject is then sensed using the buffer circuit shown in FIG. 5,allowing the impedance values to be determined and analysed by thesecond processing system 17.

Using the second processing system 17 allows the majority of processingto be performed using custom configured hardware. This has a number ofbenefits.

Firstly, the use of an second processing system 17 allows the customhardware configuration to be adapted through the use of appropriatefirmware. This in turn allows a single measuring device to be used toperform a range of different types of analysis.

Secondly, this vastly reduces the processing requirements on the firstprocessing system 10. This in turn allows the first processing system 10to be implemented using relatively straightforward hardware, whilststill allowing the measuring device to perform sufficient analysis toprovide interpretation of the impedance. This can include for examplegenerating a “Wessel” plot, using the impedance values to determineparameters relating to cardiac function, as well as determining thepresence or absence of lymphoedema.

Thirdly, this allows the measuring device 1 to be updated. Thus forexample, if an improved analysis algorithms is created, or an improvedcurrent sequence determined for a specific impedance measurement type,the measuring device can be updated by downloading new firmware viaflash memory (not shown) or the external interface 23.

It will be appreciated that in the above examples, the processing isperformed partially by the second processing system 17, and partially bythe first processing system 10. However, it is also possible forprocessing to be performed by a single element, such as an FPGA, or amore generalised processing system.

As the FPGA is a custom processing system, it tends to be more efficientin operation than a more generic processing system. As a result, if anFPGA alone is used, it is generally possible to use a reduced overallamount of processing, allowing for a reduction in power consumption andsize. However, the degree of flexibility, and in particular, the rangeof processing and analysis of the impedance which can be performed islimited.

Conversely, if only a generic processing system is used, the flexibilityis enhanced at the expensive of a decrease in efficiency, and aconsequent increase in size and power consumption.

Accordingly, the above described example strikes a balance, providingcustom processing in the form of an FPGA to perform partial processing.This can allow for example, the impedance values to be determined.Subsequent analysis, which generally requires a greater degree offlexibility can then be implemented with the generic processing system.

A further disadvantage of utilising an FPGA alone is that it complicatesthe process of updating the processing, for example, if improvedprocessing algorithms are implemented.

Electrode Connections

An example of an electrode connection apparatus is shown in FIGS. 7A and7B.

In particular, in this example, the connector includes circuitryprovided on a substrate such as a PCB (Printed Circuit Board) 61, whichis in turn mounted in a housing 60 as shown. The housing 60 includes anarm 62 which is urged toward a contact 63 provided on the substrate 61.The substrate 61 is then coupled to a respective one of the ADCs 37, 38or the DAC 39, via appropriate leads shown generally at L, such as theleads 41, 42, 53, 54.

In use, the connector couples to a conductive electrode substrate 65,such as a plastic coated in silver, and which in turn has a conductivegel 64, such as silver/silver chloride gel thereon. The arm 62 urges theconductive electrode substrate 65 against the contact 63, therebyelectrically coupling the conductive gel 64 to the circuit provided onthe substrate 61.

This ensures good electrical contact between the measuring device 1 andthe subject S, as well as reducing the need for leads between theelectrodes 13, 14 and the input of the voltage buffers, removing therequirement for additional leads, which represents an expense, as wellas a source of noise within the apparatus.

In this example, the edges and corners of the housing 60, the arm 62 andthe substrate 65 are curved. This is to reduce the chance of a subjectbeing injured when the connector is attached to the electrode. This isof particular importance when using the electrodes on lymphodemasuffers, when even a small nip of the skin can cause severecomplications.

To further enhance the useability of the housing, the housing may beformed from a material that has a low coefficient of friction and/or isspongy or resilient. Again, these properties help reduce the likelihoodof the subject being injured when the housing is coupled to theelectrode.

Electrical Isolation

A further development of the apparatus will now be described withreference to FIG. 8.

In this example, the second processing system 17 is formed from tworespective FPGA portions 17A, 17B. The two FPGA portions 17A, 17B areinterconnected via an electrically isolated connection shown generallyby the dotted line 17C. The electrically isolated connection could beachieved for example using an inductive loop connections, wireless linksor the like.

This split in the FPGA can be used to ensure that the measuring device 1is electrically isolated from the subject S. This is important forexample when taking readings with a high degree of accuracy.

In this example, the second processing system 17 will typically beimplemented such that the operation of the second FPGA portion 17B issubstantially identical for all measurement types. As a result, there isno requirement to upload firmware into the second FPGA portion 17B toallow different types of impedance analysis.

In contrast to this, the first FPGA portion 17A will typically implementfirmware depending on the impedance measurement type in a mannersubstantially as described above.

It will therefore be appreciated that this provides a mechanism by whichthe measuring device 1 is electrically isolated from the subject, whilststill allowing the benefits of use of the second processing system 17 tobe achieved.

Alternatively, equivalent electrical isolation can be obtained byproviding a single FPGA electrically isolated from the first processingsystem 10.

In this example, the second FPGA portion 17B can be provided into asubject unit, shown generally at 2, which includes the lead connections.

This allows a single measuring device 1 to communicate with a number ofdifferent subject units, each of which is associated with a respectivesubject S. This allows the measuring device 1 to provide centralisedmonitoring of a number of different subjects via way of a number ofsubject units 2. This in turn allows a number of subjects to be analysedin sequence without having to reconnect each subject S each time ananalysis is to be performed.

Lead Calibration

To assist in interpreting the impedance measurements, it is useful totake into account electrical properties of the connecting leads andassociated circuitry.

To achieve this, the leads and corresponding connections can be encodedwith calibration information. This can include, for example, usingspecific values for respective ones of the resistors in the currentsource, or buffer circuits shown in FIGS. 4 and 5. Thus for example, thevalue of the resistors R₁₂, R₁₃, R₂₆ can be selected based on theproperties of the corresponding leads.

In this instance, when the leads are connected to the measuring device1, via the corresponding ADCs 37, 38, the processing modules 32, 33 canbe to interrogate the circuitry using appropriate polling signals tothereby determine the value of corresponding resistor. Once this valuehas been determined, the second processing system 17 can use this tomodify the algorithm used for processing the voltage and current signalsto thereby ensure correct impedance values are determined.

In addition to this, the resistance value can also act as a leadidentifier, to allow the measuring device to identify the leads andensure that only genuine authorised leads are utilised. Thus, forexample, if the determined resistance value does not correspond to apredetermined value this can be used to indicate that non-genuine leadsare being used. In this instance, as the lead quality can have an effecton the accuracy of the resultant impedance analysis, it may desirable toeither generate an error message or warning indicating that incorrectleads are in use. Alternatively, the second processing system 17 can beadapted to halt processing of the measured current and voltage signals.This allows the system to ensure that only genuine leads are utilised.

This can further be enhanced by the utilisation of a unique identifierassociated with each lead connection circuit. In this instance, a uniqueidentifier can be encoded within an IC provided as part of the currentsource or voltage buffer circuits. In this instance, the measuringdevice 1 interrogates the unique identifier and compared to uniqueidentifiers stored either in local memory, or in a central database,allowing genuine leads to be identified.

This process can also be used to monitor the number of times a lead hasbeen used. In this instance, each time a lead is used, data reflectinglead usage is recorded. This allows the leads to have a predesignateduse quota life span, and once the number of times the lead is usedreaches the quota, further measurements using the leads can beprevented. Similarly, a temporal limitation can be applied by providingan expiry date associated with the lead. This can be based on the datethe lead is created, or first used depending on the preferredimplementation.

It will be appreciated that when recording lead usage, issues may ariseif this is recorded locally. In particular, this could allow a lead tobe re-used with a different measuring device. To avoid this, the leadscan be configured with a ID which is set by the measuring device onfirst use. This can be used to limit usage of the leads to a singlemeasuring device.

This can be used to ensure that the leads are correctly replaced inaccordance with a predetermined lifespan thereby helping to ensureaccuracy of measure impedance values.

Multiple Channel

A further variation to the apparatus is shown in FIG. 9.

In this example, the apparatus is adapted to provide multiple channelfunctionality allowing different body segments to undergo impedanceanalysis substantially simultaneously. In this instance, this isachieved by providing first and second processing modules 32A, 32B, 33A,33B, 34A, 34B, first and second ADCs and DACs 37A, 37B, 38A, 38B, 39A,39B as well as first and second voltage and current circuits 11A, 11B,12A, 12B, in parallel, as shown.

Thus, the measuring device 1 includes two separate impedance measuringchannels indicated by the use of reference numerals A, B. In thisinstance, this allows electrodes to be attached to body segments, suchas different limbs, with measurements being taken from each segmentsubstantially simultaneously.

As an alternative to the above described arrangement, multiple channelscould alternatively be implemented by utilising two separate secondprocessing modules 17, each one being associated with a respectivechannel. Alternatively, the signals applied to each channel could beapplied via multiplexers positioned between the ADCs 37, 38 and the DAC39 and the electrodes.

It will be appreciated that whilst two channels are shown in the aboveexample, this is for clarity only, and any number of channels may beprovided.

Switching Arrangement

FIG. 10 shows an example of an impedance measuring apparatus including aswitching arrangement. In this example, the measuring device 1 includesa switching device 18, such as a multiplexer, for connecting the signalgenerator 11 and the sensor 12 to the leads L. This allows the measuringdevice 1 to control which of the leads L are connected to the signalgenerator 11 and the sensor 12.

In this example, a single set of leads and connections is shown. Thisarrangement can be used in a number of ways. For example, by identifyingthe electrodes 13, 14, 15, 16 to which the measuring device 1 isconnected, this can be used to control to which of the leads L signalsare applied, and via which leads signals can be measured. This can beachieved either by having the user provide an appropriate indication viathe input device 22, or by having the measuring device 1 automaticallydetect electrode identifiers, as will be described in more detail below.

Alternatively, however the arrangement may be used with multiple leadsand electrodes to provide multi-channel functionality as describedabove.

Electrode Configuration

An example of an alternative electrode configuration will now bedescribed with reference to FIGS. 11A and 11B.

In this example, the electrode connector is formed from a housing 1100having two arms 1101, 1102 arranged to engage with an electrodesubstrate 1105 to thereby couple the housing 1100 to the substrate 1105.A contact 1103 mounted on an underside of the arm 1102, is urged intocontact and/or engagement with an electrode contact 1104 mounted on asurface of the electrode substrate 1105. The electrode also includes aconductive gel 1106, such as a silver/silver chloride gel, electricallyconnected to the contact 1104. This can be achieved, either by using aconductive track, such as a silver track, or by using a conductivesubstrate such as plastic coated in silver.

This allows the lead L to be electrically connected to the conductivegel 1106, allowing current to be applied to and/or a voltage measuredfrom the subject S to which they are attached. It will be appreciatedthat in this example the above described housing 1100 may also containthe buffer circuit 50, or all or part of the current source circuitshown in FIG. 4, in a manner similar to that described above withrespect to FIG. 7.

Alternatively more complex interconnections may be provided to allow themeasuring device 1 to identify specific electrodes, or electrode types.

This can be used by the measuring device 1 to control the measurementprocedure. For example, detection of an electrode type by the processingsystem 2 may be used to control the measurements and calculation ofdifferent impedance parameters, for example to determine indicators foruse in detecting oedema, monitoring cardiac function, or the like.

Similarly, electrodes can be provided with visual markings indicative ofthe position on the subject to which the electrode should be attached.For example a picture of a left hand can be shown if the electrode padis to be attached to a subject's left hand. In this instance,identification of the electrodes can be used to allow the measuringdevice 1 to determine where on the subject the electrode is attached andhence control the application and measurement of signals accordingly.

An example of this will now be described with reference to FIGS. 11C to11G. In this example the contact 1103 is formed from a contact substrate1120, such as a PCB, having a number of connector elements 1121, 1122,1123, 1124, formed from conductive contact pads, typically made ofsilver or the like. The connector elements are connected to the lead Lvia respective electrically conductive tracks 1126, typically formedfrom silver, and provided on the contact substrate 1120. The lead Lincludes a number of individual wires, each electrically coupled to arespective one of the connector elements 1121, 1122, 1123, 1124.

In this example the electrode contact 1104 on the electrode substrate1105 typically includes an electrode contact substrate 1130, includingelectrode connector elements 1131, 1132, 1133, 1134, typically formedfrom silver contact pads or the like. The electrode connector elements1131, . . . 1134 are positioned so that, in use, when the electrodeconnector 1100 is attached to an electrode, the connector elements 1121. . . 1124 contact the electrode connector elements 1131, . . . 1134 toallow transfer of electrical signals with the measuring device 1.

In the examples, of FIGS. 11D to 11G, the connector element 1131 isconnected to the conductive gel 1106, via an electrically conductivetrack 1136, typically a silver track that extends to the underside ofthe electrode substrate 1105. This can be used by the measuring device 1to apply a current to, or measure a voltage across the subject S.

Additionally, selective ones of the connector elements 1132, 1133, 1134are also interconnected in four different arrangements by respectiveconnectors 1136A, 1136B, 1136C, 1136D. This allows the measuring device1 to detect which of the electrode contacts 1122, 1123, 1124 areinterconnected, by virtue of the connectors, 1136A, 1136B, 1136C, 1136D,with the four different combinations allowing the four differentelectrodes to be identified.

Accordingly, the arrangement of FIGS. 11D to 11G can be used to providefour different electrodes, used as for example, two current supply 13,14 and two voltage measuring electrodes 15, 16.

In use, the measuring device 1 operates by having the second processingsystem 17 cause signals to be applied to appropriate wires within eachof the leads L, allowing the conductivity between the connectingelements 1122, 1123, 1124, to be measured. This information is then usedby the second processing system 17 to determine which leads L areconnected to which of the electrodes 13, 14, 15, 16. This allows thefirst processing system 10 or the second processing system 17 to controlthe multiplexer 18 in the example of FIG. 10, to correctly connect theelectrodes 13, 14, 15, 16 to the signal generator 11, or the signalsensor 12.

In this example, the individual applying the electrode pads to thesubject can simply position the electrodes 13, 14, 15, 16 on the subjectin the position indicated by visual markings provided thereon. Leads maythen be connected to each of the electrodes allowing the measuringdevice 1 to automatically determine to which electrode 13, 14, 15, 16each lead L connected and then apply current signals and measure voltagesignals appropriately. This avoids the complexity of ensuring thecorrect electrode pads are connected via the correct leads L.

It will be appreciated that the above described process allows electrodeidentification simply by applying currents to the electrode connector.However, other suitable identification techniques can be used, such asthrough the use of optical encoding. This could be achieved for example,by providing a visual marker, or a number of suitably arranged physicalmarkers on the electrode connector 1104, or electrode substrate 1105.These could then be detected using an optical sensor mounted on theconnector 1100, as will be appreciated by persons skilled in the art.

Alternatively, the identifier for the electrodes may be identified by anencoded value, represented by, for example, the value of a component inthe electrode, such as a resistor or capacitor. It will therefore beappreciated that this can be achieved in a manner similar to thatdescribed above with respect to lead calibration.

An example of an alternative electrode configuration will now bedescribed with reference to FIGS. 12A to 12F. In this particular examplethe electrode is a band electrode 1200, which includes a number ofseparate electrodes. In this example the electrode is formed from anelongate substrate 1210 such as a plastic polymer coated with shieldingmaterial and an overlaying insulating material.

A number of electrically conductive tracks 1220 are provided on thesubstrate extending from an end of the substrate 1211 to respectiveconductive contact pads 1230, spaced apart along the length of thesubstrate in sequence. This allows a connector similar to the connectorsdescribed above, but with corresponding connections, to be electricallycoupled to the tracks 1220.

The tracks 1220 and the contact pads 1230 may be provided on thesubstrate 1210 in any one of a number of manners, including for example,screen printing, inkjet printing, vapour deposition, or the like, andare typically formed from silver or another similar material. It will beappreciated however that the tracks and contact pads should be formedfrom similar materials to prevent signal drift.

Following the application of the contact pads 1230 and the tracks 1220,an insulating layer 1240 is provided having a number of apertures 1250aligned with the electrode contact pads 1230. The insulating layer istypically formed from a plastic polymer coated with shielding materialand an overlaying insulating material.

To ensure adequate conduction between the contact pads 1230, and thesubject S, it is typical to apply a conductive gel 1260 to the contactpads 1230. It will be appreciated that in this instance gel can beprovided into each of the apertures 1250 as shown.

A removable covering 1270 is then applied to the electrode, to maintainthe electrode's sterility and/or moisture level in the gel. This may bein the form of a peel off strip or the like which when removed exposesthe conductive gel 1260, allowing the electrode to be attached to thesubject S.

In order to ensure signal quality, it is typical for each of the tracks1220 to comprise a shield track 1221, and a signal track 1222, as shown.This allows the shield on the leads L, such as the leads 41, 42, 51 tobe connected to the shield track 1221, with the lead core being coupledto the signal track 1222. This allows shielding to be provided on theelectrode, to help reduce interference between applied and measuredsignals.

This provides a fast straight-forward and cheap method of producing bandelectrodes. It will be appreciated that similar screen printingtechniques may be utilised in the electrode arrangements shown in FIGS.7A and 7B, and 11A-11G.

The band electrode may be utilised together with a magnetic connector aswill now be described with respect to FIGS. 12G and 12H. In thisexample, the band electrode 1200 includes two magnets 1201A, 1201Bpositioned at the end 1211 of the substrate 1210. The connector, isformed from a connector substrate 1280 having magnets 1281A, 1281Bprovided therein. Connecting elements 1282 are also provided, and thesewould in turn be connected to appropriate leads L.

The magnets 1201A, 1281A; 1201B (not shown for clarity), 1281B can bearranged to align and magnetically couple, to urge the connectorsubstrate 1280 and the band electrode 1200 together. Correct alignmentof the poles of the magnets 1201A, 1281A; 1201B, 1281B can also be usedto ensure both the correct positioning and orientation of the connectorsubstrate 1280 and band electrode, which can ensure correct alignment ofthe connecting elements 1282, with corresponding ones of the tracks1220, on the band electrode 1200.

It will be appreciated that this can be used to ensure correctconnection with the electrode, and that a similar magnetic alignmenttechnique may be used in the connectors previously described.

In use, the band electrode may be attached to the subject's torso, asshown in FIG. 121. The electrode will typically include an adhesivesurface, allowing it to stick to the subject. However, a strap 1280 mayalso be used, to help retain the electrode 1200 in position. Thisprovides an electrode that is easy to attach and position on thesubject, and yet can be worn for an extended period if necessary. Theband electrode 1200 may also be positioned on the subject at otherlocations, such as on the side of the subject's torso, or laterallyabove the naval, as shown.

The band electrode 1200 provides sufficient electrodes to allow cardiacfunction to be monitored. In the above example, the band electrodeincludes six electrodes, however any suitable number may be used,although typically at least four electrodes are required.

Variable Current

A further feature that can be implemented in the above measuring deviceis the provision of a signal generator 11 capable of generating avariable strength signal, such as a variable current. This may be usedto allow the measuring device 1 to be utilised with different animals,detect problems with electrical connections, or to overcome noiseproblems.

In order to achieve this, the current source circuit shown in FIG. 4 ismodified as shown in FIG. 13. In this example, the resistor R₁₀ in thecurrent source circuit of FIG. 4 is replaced with a variable resistorVR₁₀. Alteration of the resistance of the resistor VR₁₀ will result in acorresponding change in the magnitude of the current applied to thesubject S.

To reduce noise and interference between the current source circuit andthe control, which is typically achieved using the second processingmodule 17, it is typical to electrically isolate the variable resistor17 from the control system. Accordingly in one example, the variableresistor VR₁₀ is formed from a light dependent resistor. In thisexample, an light emitting diode (LED) or other illumination source canbe provided, as shown at L₁. The LED L₁ can be coupled to a variablepower supply P of any suitable form. In use, the power supply P, iscontrolled by the second processing module 17, thereby controlling theintensity of light generated by the LED L₁, which in turn allows theresistance VR₁₀, and hence the applied current, to be varied.

In order to operate the measuring device 1, the first processing system10 and the second processing system 17 typically implement the processdescribed in FIG. 14. In this example, at step 1400 the user selects ameasurement or an animal type utilising the input/output device 22.

At step 1410 the first processing system 10 and the second processingsystem 17 interact to determine one or more threshold values based onthe selected measurement or animal type. This may be achieved in any oneof a number of ways, such as by having the first processing system 10retrieve threshold values from the memory 21 and transfer these to thesecond processing system 17, although any suitable mechanism may beused. In general, multiple thresholds may be used to specify differentoperating characteristics, for signal parameters such as a maximumcurrent that can be applied to the subject S, the minimum voltagerequired to determine an impedance measurement, a minimum signal tonoise ratio, or the like.

At step 1420 the second processing system 17 will activate the signalgenerator 11 causing a signal to be applied to the subject S. At step1430 the response signal at the electrodes 15,16 is measured using thesensor 12 with signals indicative of the signal being returned to thesecond processing system 17 at step 1430.

At step 1440 the second processing system 17 compares the at least oneparameter of the measured signal to a threshold to determine if themeasured signal is acceptable at step 1450. This may involve for exampledetermining if the signal to noise levels within the measured voltagesignal are above the minimum threshold, or involve to determine if thesignal strength is above a minimum value.

If the signal is acceptable, impedance measurements can be performed atstep 1460. If not, at step 1470 the second processing system 17determines whether the applied signal has reached a maximum allowable.If this has occurred, the process ends at step 1490. However, if themaximum signal has not yet been reached, the second processing system 17will operate to increase the magnitude of the current applied to thesubject S at step 1480 before returning to step 1430 to determine a newmeasured signal.

Accordingly, this allows the current or voltage applied to the subject Sto be gradually increased until a suitable signal can be measured toallow impedance values to be determined, or until either a maximumcurrent or voltage value for the subject is reached.

It will be appreciated that the thresholds selected, and the initialcurrent applied to the subject S in step 1420 will typically be selecteddepending on the nature of the subject. Thus, for example, if thesubject is a human it is typical to utilise a lower magnitude currentthan if the subject is a animal such as a mouse or the like.

Device Updates

An example of a process for updating the measuring device will now bedescribed with reference to FIG. 15.

In one example, at step 1500 the process involves determining ameasuring device 1 is to be configured with an upgrade, or the like,before configuration data is created at step 1510. At step 1520 theconfiguration data is typically uploaded to the device before the deviceis activated at 1530. At 1540 when the device commences operation theprocessing system 2 uses the configuration data to selectively activatefeatures, either for example by controlling the upload of instructions,or by selectively activating instructions embedded within the processingsystem 2 or the controller 19.

This can be achieved in one of two ways. For example, the configurationdata could consist of instructions, such as a software or firmware,which when implemented by the processing system 2 causes the feature tobe implemented. Thus, for example, this process may be utilised toupdate the operation of the firmware provided in the second processingsystem 17, the processing system 10 or the controller 19 to allowadditional functionality, improved measuring algorithms, or the like, tobe implemented.

Alternatively, the configuration data could be in the form of a list offeatures, with this being used by the processing system 2 to accessinstructions already stored on the measuring device 1. Utilisation ofconfiguration data in this manner, allows the measuring device to beloaded with a number of as yet additional features, but non-operationalfeatures, when the device is sold. In this example, by updating theconfiguration data provided on the measuring device 1, this allows thesefurther features to be implemented without requiring return of themeasuring device 1 for modification.

This is particularly useful in the medical industry as it allowsadditional features to be implemented When the feature receives approvalfor use. Thus, for example, techniques may be available for measuring ordetecting lymphoedema in a predetermined way, such as through the use ofa particular analysis of measured voltage signals or the like. In thisinstance when a device is sold, approval may not yet have been obtainedfrom an administering body such as the Therapeutic Goods Administration,or the like. Accordingly, the feature is disabled by appropriate use ofa configuration data. When the measurement technique subsequently gainsapproval, the configuration data can be modified by uploading a newupdated configuration data to the measuring device, allowing the featureto be implemented.

It will be appreciated that these techniques may be used to implementany one of a number of different features, such as different measuringtechniques, analysis algorithms, reports on results of measuredimpedance parameters, or the like.

An example of a suitable system for providing updates will now bedescribed with respect to FIG. 16. In this example, a base station 1600is coupled to a number of measuring devices 1, and a number of endstations 1603 via a communications network 1602, such as the Internet,and/or via communications networks 1604, such as local area networks(LANs), or wide area networks (WANs). The end stations are in turncoupled to measuring devices 1, as shown.

In use, the base station 1600 includes a processing system 1610, coupledto a database 1611. The base station 1600 operates to determine whenupdates are required, select the devices to which updates are applied,generate the configuration data and provide this for update to thedevices 1. It will be appreciated that the processing system 1610 maytherefore be a server or the like.

This allows the configuration data to be uploaded from the server eitherto a user's end station 1603, such as a desk top computer, lap top,Internet terminal or the like, or alternatively allows transfer from theserver via the communications network 1602, 1604, such as the Internet.It will be appreciated that any suitable communications system can beused such as wireless links, wi-fi connections, or the like.

In any event, an example of the process of updating the measuring device1 will now be described in more detail with reference to FIG. 17. Inthis example, at step 1700 the base station 1600 determines that thereis a change in the regulatory status of features implemented within acertain region. As mentioned above this could occur for examplefollowing approval by the TGA of new features.

The base station 1600 uses the change in regulatory status to determinenew features available at step 1710, before determining an identifierassociated with each measuring device 1 to be updated at step 1720. Aschanges in regulatory approval are region specific, this is typicallyachieved by having the base station 1600 access database 1611 includingdetails of the regions in which each measuring device sold are used. Thedatabase 1611 includes the identifier for each measuring device 1,thereby allowing the identifier of each measuring device to be updatedto be determined.

At step 1730, the base station 1600 determines the existingconfiguration data, typically from the database 1611, for a next one ofthe measuring devices 1, before modifying the configuration data toimplement the new features at step 1740. The configuration data is thenencrypted utilising a key associated with the identifier. The key may beformed from a unique prime number associated with the serial number, orpartially derived from the serial number, and is typically stored in thedatabase 1611, or generated each time it is required using apredetermined algorithm.

At step 1760 the encrypted configuration data is transferred to themeasuring device 1 as described above.

At step 1770 when the device restarts and the first processing system 10is activated, the first processing system 10 determines the encryptionkey, and uses this to decrypt the configuration data.

This may be achieved in any one of a number of ways, such as bygenerating the key using the serial number or other identifier, and apredetermined algorithm. Alternatively, this may be achieved byaccessing a key stored in the memory 21. It will be appreciated that anyform of encryption may be used, although typically strong encryption isused, in which a secret key is used to both encrypt and decrypt theconfiguration data, to thereby prevent fraudulent alteration of theconfiguration by users, as will be explained in more detail below.

At step 1780, the first processing system 10 activates software featureswithin the second processing system 17 using the decrypted configurationdata.

It will therefore be appreciated that this provides a mechanism forautomatically updating the features available on the measuring device.This may be achieved either by having the second processing system 17receive new firmware from the processing system 10, or by activatingfirmware already installed on the second processing system 17, asdescribed above.

As an alternative to performing this automatically when additionalfeatures are approved for use, the process can be used to allow featuresto be activated on payment of a fee. In this example, a user maypurchase a measuring device 1 with limited implemented functionality. Bypayment of a fee, additional features can then be activated as and whenrequired by the user.

In this example, as shown in FIG. 18, when the user selects an inactivefeature at step 1800, the first processing system 10 will generate anindication that the feature is unavailable at step 1810. This allows theuser to select an activate feature option at step 1820, which typicallyprompts the user to provide payment details at step 1830. The paymentdetails are provided to the device manufacturer in some manner and mayinvolve having the user phone the device manufacturer, or alternativelyenter the details via a suitable payment system provided via theInternet or the like.

At step 1840, once the payment is verified, the process can move to step1720 to allow an automatic update to be provided in the form of asuitable configuration data. However, if payment details are notverified the process ends at 1850.

It will be appreciated by a person skilled in the art that encryptingthe configuration data utilising a unique identifier means that theconfiguration data received by a measuring device 1 is specific to thatmeasuring device. Accordingly, the first processing system 10 can onlyinterpret the content of a configuration data if it is both encryptedand decrypted utilising the correct key. Accordingly, this preventsusers exchanging configuration data, or attempting to re-encrypt adecrypted file for transfer to a different device.

It will be appreciated that in addition to, or as an alternative tosimply specifying features in the configuration data, it may benecessary to upload additional firmware to the second processing system17. This can be used for example, to implement features that could notbe implemented using the firmware shipped with the measuring device 1.

In this example, it would be typical for the configuration data toinclude any required firmware to be uploaded, allowing this to be loadedinto the second processing system 17, using the first processing system10. This firmware can then either be automatically implemented, orimplemented in accordance with the list of available features providedin the configuration data.

It will be appreciated that this provides a mechanism for updatingand/or selectively activating or deactivating features, such asmeasuring protocols, impedance analysis algorithms, reports interpretingmeasured results, or the like. This can be performed to ensure themeasuring device conforms to existing TGA or FDA approvals, or the like.

Housing

In order to provide a housing configuration with suitable electricalisolation for the subject an arrangement similar to that shown in FIG.19 can be used.

In this example the measuring device 1 is provided in a housing 70 whichincludes a touch screen 71, forming the I/O device 22, together withthree respective circuit boards 72, 73, 74. In this instance the digitalelectronics including the second processing system 17 and the firstprocessing system 10 are provided on the circuit board 72. The circuitboard 73 is an analogue circuit board and includes the ADCs 37, 38, theDAC 39. A separate power supply board is then provided at 74. The supplyboard typically includes an integrated battery, allowing the measuringdevice 1 to form a portable device.

It is also typical housing electrical/magnetic shielding from theexternal environment, and accordingly, the housing is typically formedfrom a mu-metal, or from aluminium with added magnesium.

Persons skilled in the art will appreciate that numerous variations andmodifications will become apparent. All such variations andmodifications which become apparent to persons skilled in the art,should be considered to fall within the spirit and scope that theinvention broadly appearing before described.

Thus, for example, it will be appreciated that features from differentexamples above may be used interchangeably where appropriate.Furthermore, whilst the above examples have focussed on a subject suchas a human, it will be appreciated that the measuring device andtechniques described above can be used with any animal, including butnot limited to, primates, livestock, performance animals, such racehorses, or the like.

The above described processes can be used for diagnosing the presence,absence or degree of a range of conditions and illnesses, including, butnot limited to oedema, lymphodema, body composition, or the like.

It will also be appreciated above described techniques, such aselectrode identification, device updates and the like may be implementedusing devices that do not utilise the separate first processing system10 and second processing system 17, but rather use a single processingsystem 2, or use some other internal configuration.

Additionally, the end station 1603 can effectively perform any one ormore of tasks performed by the first processing system 10 in theexamples throughout the specification. Accordingly, the device could beprovided without the first processing system 10, with the functionalityusually performed by the first processing system 10 being performed byan end station 1603. In this arrangement, the end station 1603 thereforeeffectively forms part or all of the first processing system 10. Thisallows the measuring device 1 to be provided including only the secondprocessing system 17 coupled directly to the external interface 23 toallow the measuring device 1 to be controlled by the end station 1603.This would typically be achieved via the use of suitable applicationssoftware installed on the end station 1603.

The invention claimed is:
 1. Apparatus for performing impedancemeasurements on a subject, the apparatus including: a) a firstprocessing system for: i) determining an impedance measurementprocedure; and, ii) selecting instructions corresponding to themeasurement procedure; and, b) a second processing system for: i)generating, using the instructions, control signals, the control signalsbeing used to apply one or more signals to the subject; ii) receiving anindication of the one or more signals applied to the subject; iii)receiving an indication of one or more signals measured across thesubject; iv) performing, using the instructions, at least preliminaryprocessing of the indications to thereby allow impedance values to bedetermined.
 2. Apparatus according to claim 1, wherein the apparatustransfers the instructions from the first processing system to thesecond processing system.
 3. Apparatus according to claim 1, wherein theapparatus selects the instructions using configuration data. 4.Apparatus according to claim 3, wherein the apparatus receives theconfiguration data from a remote processing system.
 5. Apparatusaccording to claim 1, wherein the instructions are in the form of atleast one of: a) firmware; and, b) embedded systems.
 6. Apparatusaccording to claim 1, wherein the second processing system is an FPGA.7. Apparatus according to claim 1, wherein the apparatus includes aninput device, and wherein the first processing system is coupled to theinput device to thereby determine the impedance measurement procedure inaccordance with input commands from an operator.
 8. Apparatus accordingto claim 1, wherein the first processing system includes a store forstoring at least one profile, the at least one profile representing apredetermined impedance measurement procedure.
 9. Apparatus according toclaim 1, wherein the control signals represent a sequence ofpredetermined electrical signals, the sequence being dependent on theselected impedance measurement type.
 10. Apparatus according to claim 1,wherein the apparatus includes: a) a current ADC for: i) receivingsignals from a current circuit; and, ii) providing the indication of theone or more signals applied to the subject to the second processingsystem; and, b) a voltage ADC for: i) receiving signals from a voltagecircuit; and, ii) providing the indication of the one or more signalsmeasured from the subject to the second processing system.
 11. Apparatusaccording to claim 10, wherein the apparatus includes at least onebuffer circuit for: a) receiving voltage signals from a voltageelectrode; b) filtering and amplifying the voltage signals; and, c)transferring the filtered and amplified voltage signals to the voltageADC via a differential amplifier.
 12. Apparatus according to claim 10,wherein the apparatus includes a current source circuit for: a)receiving one or more control signals; b) filtering and amplifying thecontrol signals to thereby generate one or more current signals; c)applying the current signals to a current electrode; and, d)transferring an indication of the applied signals to the current ADC.13. Apparatus according to claim 1, wherein the apparatus includes acontrol signal DAC for: a) receiving the control signals from the secondprocessing system; and, b) providing analogue control signals to acurrent circuit to thereby cause one or more current signals to beapplied to the subject in accordance with the control signals. 14.Apparatus according to claim 1, wherein the second processing system isformed from first and second processing system portions, the first andsecond processing system portions being electrically isolated to therebyelectrically isolate the subject from the first processing system. 15.Apparatus according to claim 1, wherein the apparatus includes: a) ameasuring device including at least the first processing system; and, b)one or more subject units, each subject unit including at least part ofthe second processing system.
 16. Apparatus according to claim 1,wherein the apparatus includes at least two current electrodes forapplying current signals to the subject, and a switch connected to thecurrent electrodes for discharging the subject prior to measuring theinduced voltage.
 17. Apparatus according to claim 1, wherein theapparatus includes a housing having: a) a display; b) a first circuitboard for mounting at least one of the processing systems; c) a secondcircuit board for mounting at least one of an ADC and a DAC; and, d) athird circuit board for mounting a power supply.
 18. Apparatus accordingto claim 17, wherein the housing is formed from at least one of amu-metal and aluminium with added magnesium, to thereby provideelectrical/magnetic shielding.
 19. Apparatus according to claim 1,wherein the apparatus includes multiple channels, each channel being forperforming impedance measurements using a respective set of electrodes.20. Apparatus according to claim 1, wherein the apparatus is for: a)determining an electrode identifier associated with at least oneelectrode provided on the subject; b) determining, using the electrodeidentifier, an electrode position indicative of the position of the atleast one electrode on the subject; and, c) performing at least oneimpedance measurement using the electrode position.
 21. Apparatusaccording to claim 1, wherein the apparatus is for: a) determining aparameter associated with at least one electrode lead; and, b) causingat least one impedance measurement to be performed using the determinedparameter.
 22. Apparatus according to claim 1, wherein the apparatus isfor: a) receiving configuration data, the configuration data beingindicative of at least one feature; b) determining, using theconfiguration data, instructions representing the at least one feature;and, c) causing, using the instructions, at least one of: i) at leastone impedance measurement to be performed; and, ii) at least oneimpedance measurement to be analysed.
 23. Apparatus according to claim1, wherein the apparatus is for: a) causing a first signal to be appliedto the subject; b) determining at least one parameter relating to atleast one second signal measured across the subject; c) comparing the atleast one parameter to at least one threshold; and, d) depending on theresults of the comparison, selectively repeating steps (a) to (d) usinga first signal having an increased magnitude.
 24. A method of performingimpedance measurements on a subject, the method including: a) using afirst processing system for: i) determining an impedance measurementprocedure; and, ii) selecting instructions corresponding to themeasurement procedure; and, b) using a second processing system for: i)generating, using the instructions, control signals, the control signalsbeing used to apply one or more signals to the subject; ii) receiving anindication of the one or more signals applied to the subject; iii)receiving an indication of one or more signals measured across thesubject; iv) performing, using the instructions, at least preliminaryprocessing of the first and second data to thereby allow impedancevalues to be determined.
 25. A method of diagnosing conditions in asubject, the method including, in a processing system: a) using a firstprocessing system for: i) determining an impedance measurementprocedure; and, ii) selecting instructions corresponding to themeasurement procedure; and, b) using a second processing system for: i)generating, using the instructions, control signals, the control signalsbeing used to apply one or more signals to the subject; ii) receiving anindication of the one or more signals applied to the subject; iii)receiving an indication of one or more signals measured across thesubject; iv) performing, using the instructions, at least preliminaryprocessing of the first and second data to thereby allow impedancevalues to be determined.
 26. A method for performing impedancemeasurements on a subject, the method including: a) in a firstprocessing system: i) determining an impedance measurement procedure;and, ii) selecting instructions corresponding to the measurementprocedure; and, b) in a second processing system: i) generating, usingthe instructions, control signals, the control signals being used toapply one or more signals to the subject; ii) receiving an indication ofthe one or more signals applied to the subject; iii) receiving anindication of one or more signals measured across the subject; iv)performing, using the instructions, at least preliminary processing ofthe indications to thereby allow impedance values to be determined. 27.Apparatus for use in diagnosing conditions in a subject, the apparatusincluding: a) a first processing system for: i) determining an impedancemeasurement procedure; and, ii) selecting instructions corresponding tothe measurement procedure; and, b) a second processing system for: i)generating, using the instructions, control signals, the control signalsbeing used to apply one or more signals to the subject; ii) receiving anindication of the one or more signals applied to the subject; iii)receiving an indication of one or more signals measured across thesubject; iv) performing, using the instructions, at least preliminaryprocessing of the indications to thereby allow impedance values to bedetermined.
 28. A method for use in diagnosing conditions in a subject,the method including: a) using a first processing system for: i)determining an impedance measurement procedure; and, ii) selectinginstructions corresponding to the measurement procedure; and, b) using asecond processing system for: i) generating, using the instructions,control signals, the control signals being used to apply one or moresignals to the subject; ii) receiving an indication of the one or moresignals applied to the subject; iii) receiving an indication of one ormore signals measured across the subject; iv) performing, using theinstructions, at least preliminary processing of the first and seconddata to thereby allow impedance values to be determined.
 29. A methodfor use in diagnosing conditions in a subject, the method including: a)in a first processing system: i) determining an impedance measurementprocedure; and, ii) selecting instructions corresponding to themeasurement procedure; and, b) in a second processing system: i)generating, using the instructions, control signals, the control signalsbeing used to apply one or more signals to the subject; ii) receiving anindication of the one or more signals applied to the subject; iii)receiving an indication of one or more signals measured across thesubject; iv) performing, using the instructions, at least preliminaryprocessing of the indications to thereby allow impedance values to bedetermined.